The present invention relates to a laser beam projection device and, more particularly, to an improved arrangement for mounting a laser plasma tube in such device in a precise orientation with respect to one or more optical elements of the projection device.
A number of different laser beam projection devices have been employed in the past in surveying and construction applications to provide either a rotating reference beam of laser light or a stationary beam of laser light. A rotating reference beam of laser light defines a reference plane which may be detected at locations remote from the projection device in order to measure elevations and grades. A stationary beam of light may be used to align various construction components, such as for example sections of drainage pipe, in a straight line.
As is well known, such projection devices commonly use a helium-neon gas laser which includes a gas mixture within a glass envelope, termed a plasma tube. The laser beam emerges from the plasma tube, is directed to various optical elements, such as lenses and mirrors, and emerges from the projection device either as a stationary beam or as a rotating beam of light.
One such projection device is shown in U.S. Pat. No. 4,062,634, issued Dec. 13, 1977, to Rando et al, and assigned to the assignee of the present invention. In the Rando et al device, the plasma tube is mounted by means of clam-shell type clamps on the support frame of the device. An optics arrangement, including a pair of adjustably mounted mirrors, deflects the light beam through approximately 180.degree.. The mirrors are oriented by a technician to produce a laser beam which is aligned within requisite tolerances with a required beam path through the optical elements. A circular prism with its outer surfaces having a relatively small included angle may also be positioned in the beam path. To obtain fine adjustment of the beam orientation, the prism is rotated about its center.
One problem encountered with helium-neon lasers is that the alignment of the laser light beam with respect to the plasma tube, although constant for each individual plasma tube, varies from tube to tube. It has been common, therefore, to provide a series of adjustable optical elements in the laser projection device to align the beam properly after it emerges from the plasma tube. A difficulty arises when the plasma tube fails in such a device, the tube is removed and a replacement tube installed. Since the replacement tube produces a beam having a substantially different orientation than that produced by the original tube, a complete realignment of the adjustable optical elements is required. This process is time consuming and must be performed by a trained technician.
In order to facilitate plasma tube replacement, some projection devices have used plasma tubes which are pre-aligned within an outer casing to produce a beam aligned with the casing. With the casing mounted on and properly positioned with respect to a support frame of the laser beam projection device, the beam is then properly aligned with respect to optical elements mounted on the frame. This pre-alignment approach has not been particularly successful, however. A substantial difficulty that has been encountered in replacing such a pre-aligned plasma tube assembly is in positioning the plasma tube casing properly with respect to the support frame of the device. Additionally, such a replacement operation is typically complicated, requiring the disassembly of a large portion of the projection device.
Accordingly, it is seen that there is a need for a laser beam projection device including an arrangement for mounting the laser plasma tube in the projection device in which replacement of the plasma tube is simple and does not require substantial readjustment of optical elements.